The manner in which different pharmacologic agents utilize or depend upon cellular Ca ions sites or stores for the initiation and maintenance of contractile responses in vascular smooth muscle will be examined. Conventional techniques will be employed to measure isometric tension responses, Ca45 uptake and rapid washout, and total ionic (Ca ions, Na ion, K ion) concentrations as well as to obtain vascular smooth muscle tissue cultures and microsomal and mitochondrial preparations. Experiments will be designed to elucidate the manner in which the actions of different stimulatory and inhibitory agents depend upon cellular Ca ions sites or stores as well as the nature of Ca45 uptake and retention. This information could provide a basis for evaluation of ideas concerning the mechanisms by which membrane-bound Ca ions is involved in events leading to alterations in contractile tone. The effects of selected divalent ions (e.g., Mn ions, Cd ions) and La ions on intracellular Ca45 binding will be examined under conditions in which membrane permeability to these ions is increased. An attempt will be made to characterize the relative importance of affinity of Ca ions for binding sites and of the number of binding sites present in rabbit and rat aortic smooth muscle and in rabbit mesenteric artery. Measurement of Ca45 fluxes in vascular smooth muscle tissue cultures and subcellular (microsomal and mitochondrial) preparations will permit comparison of effects obtained with stimulatory and inhibitory agents in these systems with corresponding effects in more anatomically complex intact isolated preparations. Thus, insight concerning the cellular actions and mechanisms of action of a variety of ions and drugs in different types of vascular smooth muscle will be obtained.